The present invention relates to a piezoelectric resonator, more particularly to improvements in a piezoelectric resonator using a lithium niobate (LiNbO.sub.3) single crystal.
A piezoelectric resonator comprising a quartz (rock crystal) resonator chip is used in an oscillator circuit or a filter circuit of various electronic apparatuses. Recently, a piezoelectric resonator chip using lithium tantalate (LiTaO.sub.3) instead of quartz has been developed to obtain a small size piezoelectric resonator with a high stability, since lithium tantalate has a high electro-mechanical coupling coefficient and stable temperature characteristic. A strip type resonator element using the lithium tantalate resonator chip is known, in which the resonator chip is formed in a rectangular parallelepiped shape and an electrode is provided on the center portion of each of the upper and the lower surfaces and over the entire width of the chip, to obtain a smaller size piezoelectric resonator and minimize the generation of spurious waves. The applicant has proposed a strip type resonator element comprising a lithium tantalate resonator chip in which the crystal axes are taken into consideration so that the lithium tantalate wafer is made of a three degree rotated X-cut plate and the longitudinal direction of the resonator chip is inclined about fifty degrees with respect to the Y-axis of the wafer. By such an arrangement, the temperature characteristic is upgraded and the equivalent series resistance is lowered, so that the stability and the energy efficiency of the piezoelectric resonator can be upgraded. Such a lithium tantalate strip type resonator element can be very effectively used in an apparatus which exchanges data or signals with another apparatus, and the data or signals can be reliably introduced into the apparatus even though the two apparatuses are disposed in a different temperature environment, since the resonator of one apparatus is strictly synchronized with the resonator of the other apparatus due to the very stable temperature characteristic of that piezoelectric resonator. However, the productivity of the lithium tantalate resonator chip is limited, since a large size lithium tantalate crystal body cannot be obtained due to its crystal characteristic, which limits the size of the wafer to under approximately 3 inches.
Lithium niobate is known as a piezoelectric material with a higher electro-mechanical coupling coefficient than lithium tantalate. However, the temperature characteristic of lithium niobate is not stable when compared with lithium tantalate, and lithium niobate has not been used as a piezoelectric resonator chip material since the resonance frequency changes in response to temperature changes. At present, there are various electonic apparatuses which comprise an electronic control circuit using a microcomputer for controlling the apparatus individually. The oscillator used in such a control circuit need not be synchronized with another oscillator, since the circuit controls only the apparatus in which the circuit is housed and does not exchange data or signals with another apparatus. The parts of the apparatus are controlled by the control circuit having one oscillator as a mono source of clock signals. Such an oscillator does not require a highly stable temperature characteristic since it does not communicate with other apparatuses and a change of frequency of the clock signals does not affect the operation of the apparatus since all of the parts are controlled based on the mono source of the clock signals. Therefore, a small size oscillator having a high energy efficiency, but without the necessity of a stable temperature characteristic is now in demand.